This invention relates to electronic control circuits for automatic transfer switches. More specifically, the invention is directed to apparatus for protecting the user of electronic control circuits for automatic transfer switches from the potentially dangerous A.C. line voltage by galvanic isolation of the line voltage from the required measurement circuit.
Electronic control circuits for automatic transfer switches measure normal and emergency line voltages in order to ensure that emergency power is promptly applied upon failure of the normal source of line voltage and that the emergency voltage is a reasonable facsimile of the normal voltage. In order to protect the user of the control circuitry from the potentially dangerous line voltages, galvanic isolation between the line voltage and the measurement circuit must be maintained. The amplitudes of the line voltages being monitored must be scaled to smaller magnitudes which can be accommodated by semiconductor measuring circuits. In the past, transformers have been used to isolate the measurement circuit from the line voltages, and to scale the voltages to workable levels. Such prior art solutions have been problematic in that transformers are relatively large and heavy and must be manually mounted in their operating environments.